Bypass ratio

[PMN1]

Post WW2 but hopefully I can get some answers here.

From Bill Gunston's 'The development of jet and turbine aero engines'...

In Rolls Royce case while Griffiths rightly kept thinking of BPR around 20, a study was made to find optimum BPR for an engine in an external pod and, because of incorrect assumptions of nacelle drag, came up with the answer that it should not exceed 1.0.'

Anyone know when this study was done?

[Red Admiral]

It depends what speed you are flying at.

Propulsive efficiency = 2 / (1 + (Vplane/Vjet))

Bypass of 1.0 = Vjet of 814m/s

So to get maximum efficiency you need to fly at 814m/s or 1830mph c. M1.8

For A slower aircraft flying at 500mph for example.

max efficiency is when Vjet = 500mph = 222m/s which is a bypass ratio of around 13 or so.

It is more complicated than this but I can't remember the equations for dealing with the drag of the nacelle off the top of my head, or dimensions of a nacelle.

[montanamotor]

quote:Originally posted by PMN1

Post WW2 but hopefully I can get some answers here.

From Bill Gunston's 'The development of jet and turbine aero engines'...

In Rolls Royce case while Griffiths rightly kept thinking of BPR around 20, a study was made to find optimum BPR for an engine in an external pod and, because of incorrect assumptions of nacelle drag, came up with the answer that it should not exceed 1.0.'

Anyone know when this study was done?



Hi, PMN1,

see Convair B 58 "Hustler":

Four Nacelles, Mach 2.0 plus. BPR? No idea, I must admit. The russians also use nacelles in their supersonic swept-wing "Blinder"-bomber - Tupolev TU 22 A, as well as the later, improved Tupolev 22 M "Backfire". BPR? No idea, either.

I just wanted to throw in something...

Cheers!

Montanamotor

[Trexx]

Brainiacs. [^]


The B-47 Stratojet had 'issues' with its inboard nacelle pylon. The transonic speed range was a new animal, discovered some vortex and shock wave problems that were worked out... eventually.